Wind up here!

In "Ready, Set, Slurp!" (p. 16), you learned how chameleons use potential and kinetic energy to launch their tongues. Follow this activity to build a "Retro Roller" toy to see these energies in action.

YOU NEED:

12-oz coffee can * 1 can opener * 2 plastic, 12-oz coffee can lids * marker * 22 pennies * clear tape * 2 size-32 rubber bands * 2 pencil stubs, each 8 centimeters (3 inches) long

TO DO:

1. Use a can opener to remove the bottom of an empty coffee can. (Caution: Be careful of sharp edges.) Set can aside.

2. Find the center of two plastic coffee can lids. Then, use a marker to dot the spot. Poke a hole in the center of the lids with a pencil point. Make the holes big enough for a rubber band to fit through. Set lids aside.

3. Stack 14 pennies and wrap the entire stack in tape to hold the stack together.

4. Place one end of a size-32 rubber band on top of the stack, close to one edge, as shown in Figure A (right). (Note: If the band is set in the middle of the stack, this project won't work.) Securely tape the rubber band to the stack.

5. Flip over the stack of pennies. Place a second rubber band on top of the stack. Make sure that the rubber band is aligned directly above the rubber band that's now at the bottom of the stack. Tape the rubber band in place.

6. Add four pennies to the top of the stack so that they cover the taped part of the rubber band. Tape the pennies in place. Turn the stack over and repeat. For example, see Figure B (right).

7. Hold one lid with the top side facing upward. Then, push one of the rubber bands on the stack up through the lid's hole.

8. Slip a pencil stub through the rubber band loop that is poking through the hole. Tape the pencil to the lid to keep it from slipping.

9. Repeat Steps 7 and 8 for the other rubber band. When completed, it should look like Figure C (below).

10. Carefully bend one of the lids and push it through to the other side of the can.

11. Making sure the rubber bands are not twisted, press both lids onto the can. When completed, both sides of the can should be covered with a lid.

12. Place the can gently on the floor. Give the "Retro Roller" a push. Observe how the can moves.

CONCLUSIONS:

1. What happens to the can after you push it?

2. Consider this: The stack of pennies is heavy and does not move when the can rolls. But the rubber bands twist with each roll of the can. How did the can do its back-and-forth rolling trick? Explain in terms of energy.

TAKE IT FURTHER:

What would happen if you altered the size of the can, the length or thickness of the rubber bands, or the number of pennies in the stack. Try it to find out.

ANSWERS

1. The can rolls in one direction. Then it stops before rolling in the other direction. This back-and-forth motion repeats.

2. By pushing the can, you give the can potential energy. As the can moves forward, that energy converts into kinetic energy, As the rubber band twists, they store some of the kinetic energy as potential energy. When the can stops, the rubber bands untwist. This motion turns the potential energy into kinetic energy, roiling the can backwards.

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